He had been in discussions with the Eagles 12 months earlier about joining the club as an assistant coach once his playing career was completed. But he was contracted to the Hawks for 2017 and the Eagles never considered he could come to the club as a player.

But Mitchell confirmed on Wednesday that Hawthorn coach Alastair Clarkson had raised the idea of moving to the Eagles next year in a startlingly frank discussion on Brownlow Medal day, just 10 days after the Hawks had been bounced out of the finals series in straight sets.

Mitchell consulted his family and decided that it was the right time to move to Perth and play under Eagles coach Adam Simpson, who was an assistant coach at the Hawks between 2010 and 2013.

In a letter to Hawthorn members, Mitchell hoped fans would understand the reasons for his shock move.

"At 34, with a young family and perhaps my best footy behind me, this all makes sense and it opens doors in terms of my next dream to be a coach," he said.

"Some of you will be angry and think the club and I have made a big mistake which I understand but emotion aside, this is the right decision for myself and the football club."

The Eagles have gained a three-time All Australian midfielder to fit in alongside Brownlow medallist Matt Priddis, All Australian Andrew Gaff and 2016 best and fairest Luke Shuey.

"Clearly I'm coming over to play first and foremost to help win silverware for the football club. I'm coming for a good reason and I want success," Mitchell told westcoasteagles.com.au.

"I'm pretty passionate about the coaching and development side of the game as well, so in addition I hope I can help some of these young guys and we can make each other better players."

Eagles football manager Craig Vozzo said the club was delighted to secure Mitchell's services.

"Sam is an ultimate professional who brings a wealth of experience to our list and we see him as a huge asset, both on and off the field for our developing players," Vozzo said.

"We are excited with what he will provide our team next year and, hopefully, beyond. He has been preparing for a coaching career for some time and we will ensure he transitions smoothly into our coaching group at the end of his playing career, whenever that may be."